1. Field of the Invention
The present invention relates generally to a multi-hop relay cellular network, and in particular, to a subframe structure for flexibly controlling an action change gap (Transmit/Receive Transition Gap (TTG)/Receive/Transmit Transition Gap (RTG)) (TTG/RTG) in a multi-hop relay cellular network and an apparatus for supporting the same.
2. Description of the Related Art
In fourth-generation (4G) mobile communication system, cells having a very small radius are located to enable rapid communications and accommodate more traffic. However, it may be impossible to achieve a centralized design using current wireless network design scheme. This wireless network should be controlled and deployed in a distributed manner and actively adapt to environment changes such as joining of a new base station. To these ends, the 4-G mobile communication system requires configuration of an autonomous adaptive wireless network.
It would be necessary to adopt techniques applied to an ad-hoc network to the mobile communication system for the substantial implementation of the autonomous adaptive wireless network required by the 4-G mobile communication system. A representative example is a multi-hop relay cellular network, in which the multi-hop relay scheme applied to the ad-hoc network is introduced to the cellular network configured with a fixed base station.
Generally, in the cellular network, since communications are conducted through one direct link between a base station and a mobile station, it is easy to establish a highly reliable radio communication link between the base station and the mobile station.
However, since the network configuration has low flexibility because of the fixed base station, it is hard to provide efficient services in a radio environment, which is subject to severe change in traffic distribution or required traffic.
To overcome this shortcoming, it is possible to apply a relay scheme, which delivers data in a multi-hop manner by use of neighboring mobile stations or relay stations. The multi-hop relay scheme can rapidly reconfigure the network under the environment change and enables the efficient use of overall network resources. Also, the multi-hop relay scheme can provide the mobile station with a radio channel of better channel status by building a multi-hop relay path by way of a repeater which is placed between the base station and the mobile station. Furthermore, a high speed data channel can be provided to mobile stations, which cannot communicate with the base station in a shadow area, by means of the multi-hop relay path, to thereby expand the cell area.
FIG. 1 depicts a configuration of a general multi-hop relay cellular network.
As shown in FIG. 1, a mobile station (MS) 110 in a service area 101 of a base station (BS) 100 is connected to BS 100 through a direct link. In contrast, a MS 120 with poor channel status, which resides outside the service area 101 of BS 100, is connected to a relay link via a relay station (RS) 130.
When MSs 110 and 120 suffer poor channel status because they are outside the service area 101 of BS 100 or in a shadow area under the severe shielding by buildings, BS 100 is able to provide better radio channels to MSs 110 and 120 by means of RS 130. Accordingly, by adopting the multi-hop relay scheme, BS 100 can provide high speed data channel in the boundary area of poor channel status and expand the cell service area. In addition, the multi-hop relay cellular network has the BS-MS link, the BS-RS link, and the RS-MS link.
The multi-hop relay scheme of FIG. 1 can set a relay link using a plurality of RSs as shown in FIG. 2.
FIG. 2 depicts a configuration of a general multi-hop cellular network.
As shown in FIG. 2, a BS 201 establishes a communication link to MS 219 using a relay link formed with RSs 211, 213, 215, and 217.
That is, BS 210 is able to expand the communication link to MS 219 using a multi-hop path.
To support the multi-hop relay cellular network, a frame structure as shown in FIG. 3 is utilized.
FIG. 3 depicts a frame structure of a conventional Time Division Duplex (TDD) system. In the following explanation, the horizontal axis indicates the time domain and the vertical axis indicates the frequency domain.
As shown in FIG. 3, a frame 300 includes a downlink (DL) subframe 311 and an uplink (UL) subframe 321. The DL subframe 311 includes a DL signal, which is transmitted from the BS to the MS via RSs. UL subframe 321 includes an UL signal, which is transmitted from the MS to the BS via the RSs.
Between DL subframe 311 and UL subframe 321, there is a Transmit/Receive Transition Gap (TTG) 331, which is a guard region. Between UL subframe 321 of the i-th frame and DL subframe 321 of the (i+1)-th frame, there is a Receive/Transmit Transition Gap (RTG) 341, which is a guard region. In the TTG, the BS changes from the transmit mode to the receive mode and the MS changes from the receive mode to the transmit mode. In the RIG, the BS changes from the receive mode to the transmit mode and the MS changes from the transmit mode to the receive mode. Thus, in the TTG/RTG, the BS and the MS perform the operating mode of signal transmission or signal reception.
To support the multi-hop relay cellular network in the frame structure of the TDD system as above, a subframe structure of FIG. 4 should be used.
FIG. 4 depicts a subframe structure of a general multi-hop TDD system. The multi-hop link signal transmission is carried out by allocating different resources. The horizontal axis indicates the time domain and the vertical axis indicates the frequency domain.
As shown in FIG. 4, allocating different time slots to subframes of each hop link in sequence constitutes a subframe.
Specifically, different time slots are allocated to a first hop 401, which delivers a DL signal from a BS to an RS1 and to a second hop 403 which delivers a DL signal from the RS1 to an RS2, to constitute the subframe.
It is noted that the time slot allocated to each hop may include a single unidirectional link subframe or a super frame consisting of multiple frames.
As noted above, the multi-hop relay cellular network sequentially performs the signal transmission of the hops in the allocated time slot. In this case, the RS of each hop has to receive the signal in the previous hop and to transmit the signal to the next hop. Hence, the “action change gap” is required between the subframes. In general, the TDD frame is short in size, taking into account the feedback delay which affects Transmit Control Protocol (TCP) throughput, Automatic Repeat Request (ARQ)/H-ARQ, and closed loop control performance. Consequently, the plurality of the TTG/RTGs for the multi-hop in the short frame size results in large overhead.
The change of the TTG/RTG can be accomplished simply by the symbol size constituting the frame. However, when the frame is shortened in consideration of the feedback delay, the “action change gap” disadvantageously acts as the large overhead by the integer symbol size.